1. Field of the Invention
The present invention relates to a high-frequency power source that supplies high-frequency electric power to a load such as a plasma processor for performing plasma etching in e.g. semiconductor wafer processing.
2. Description of the Related Art
FIG. 11 illustrates a conventional high-frequency power supply system disclosed in JP-A-H05-63604. The high-frequency power supply system includes a high-frequency power source 31 for outputting high-frequency power, and an impedance-matching unit 32, connected with the high-frequency power source 31 via e.g. a coaxial cable, for matching the input impedance of the high-frequency power source 31 with the load impedance. Further, the power supply system includes a load connecting section 33 and a load L. The connecting section 33, provided by e.g. a copper plate, is connected with the impedance-matching unit 32. The load L, e.g. a plasma processor, is connected with the load connecting section 33.
The high-frequency power source 31, an apparatus for supplying the load L with high-frequency power, includes components such as a power amplification circuit and an oscillation circuit, to output required electric power to the impedance-matching unit 32.
The impedance-matching unit 32 is configured to match the input impedance, i.e. an impedance viewed from the matching unit's inputting end toward the high-frequency power source 31, with the load impedance, i.e. an impedance viewed from the matching unit's inputting end toward the load L. The matching unit improves the efficiency in output supply from the high-frequency power source 31 to the load L.
The load L is operated to process works such as semiconductor wafers or liquid crystal substrates by means of etching, CVD, etc. As noted above, an example of the load L is a plasma processor (plasma chamber) comprising a vacuum container, in which a gas is introduced to generate plasma. The high-frequency power is used to ionize the gas to generate plasma. The plasma thus obtained is used to process works such as semiconductor wafers and liquid crystal substrates.
Now, the load L has a characteristic in terms of the relationship between the output frequency f of the high-frequency power and the reflection coefficient Γ (or impedance). Specifically, as shown in FIG. 12, the reflection coefficient Γ takes a minimum value Γs at a certain output frequency fs. Further, the value changes frequently with time in the load L, due to changes in the load such as the gas sealed in the plasma processor, the pressure inside the plasma processor and so on, as indicated in a phantom line in FIG. 12. More specifically, the output frequency f at which the reflection coefficient Γ takes a minimum value shifts to an output frequency fs′, with a minimum reflection coefficient being Γs′.
The impedance-matching unit 32 shown in FIG. 11 includes: a matching circuit built with inductors L1, L2 and variable capacitors C1, C2 serving as impedance varying devices; a detector (not illustrated) which detects the high-frequency voltage V, the high-frequency current I, and the phase difference θ between the high-frequency voltage V and the high-frequency current I; and a control circuit (not illustrated) for adjusting capacitance values of the variable capacitors C1, C2. The control circuit outputs a control signal, based on which electric motors (not illustrated) are driven to adjust capacitance of the variable capacitors C1, C2.
The detector detects the high-frequency voltage V, the high-frequency current I and the phase difference θ between the voltage V and the current I. Then, these values are inputted to the control circuit, and the control circuit calculates the input impedance Z1 of the impedance-matching unit 32 based on these inputs. Based on the input impedance Z1 and impedance values Zc1, Zc2 of the variable capacitors C1, C2, the control circuit calculates the load-circuit impedance Z2 which is the impedance viewed from the output terminal of the impedance-matching unit 32 toward the load L.
Then, based on the calculated load-circuit impedance Z2, the control circuit makes adjustment on the variable capacitors C1, C2 so that the input impedance Z1 will match the output impedance Zs (e.g. 50Ω) of the high frequency power source 31, thereby matching the impedances between the high frequency power source 31 and the load L.
There is a problem, however, that it takes time, e.g. one second or so, for the impedance-matching unit 32 to complete the impedance matching operation. On the contrary, the minimum value in the reflection coefficient and the corresponding output frequency changes instantaneously in the load L. Thus, the impedance-matching unit 32 is not suitable for such an apparatus. Further, in the impedance-matching unit 32, impedance matching by adjusting capacitance values of the variable capacitors C1, C2 is accomplished by driving electric motors. Due to the moving parts of the motors, the impedance-matching unit 32 does not have a high level of maintainability and also is susceptible to breakdown. As another problem, the impedance-matching unit 32 is costly.